[hpsdr] Frequency offset using Excalibur

[Joe Martin K5SO]

Hi John, 

I have also noticed that and confirm your observations.  I want to check to see how Hermes and Angelia responded for comparison.  Unfortunately the pre-production versions of both of those boards that I have use incorrect input transistors on the ext 10 MHz inputs so I will need to change those transistors before doing more tests.  Maybe someone has a clue as to the cause of the offset.  At the moment, I don't. 

73,  Joe K5SO

On Dec 8, 2012, at 12:45 PM, John Ackermann N8UR wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> I just discovered something very odd with my Atlas-based HPSDR system (Hermes, Mercury, Penny, Excalibur, using W5WC PowerSDR 11/5/12 and latest firmware on all boards.)
> 
> There seems to be about a 9 Hz frequency offset generated somewhere in the system.  I noticed this while monitoring WWV on 10, 15, and 20 MHz using the PowerSDR phase scope, tuning the carrier in CWU mode with a 25 Hz filter to remove modulation.
> 
> The minimum phase rotation occurs when I tune about 9 Hz below the nominal carrier frequency.  When I change to CWL, the offset reverses direction.
> 
> I've tried using both GPSDO and lab-quality Rubidium frequency references plugged into Excalibur, and get the same result.  I've watched while interrupting the external reference, and see about a 12 Hz frequency shift between states, so Mercury seems to be locking to the external reference.
> 
> To verify that it's not some propagation or interference issue, I routed the GPSDO output through a bunch of attenuation into the Mercury antenna input and see the same result on the very strong 10 MHz signal.  Since both the reference and the signal are coming from the same source, there should be exactly 0 offset.
> 
> In the PowerSDR cal tab, the frequency correction is set to 0.  When I run the frequency calibration, the correction factor remains at zero -- does this mean that PowerSDR think the frequency is accurate and the issue is just in the phase scope?
> 
> Either I'm misunderstanding or misconfiguring something for this test, or there's some sort of offset being generated in the hardware or software.  Anyone have any suggestions?
> 
> Thanks,
> 
> John
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